Terpolymers derived from a vinyl aromatic monomer, an acrylic nitrile and an alkylacrylate or alkylmethacrylate are known processing aids for vinylhalide (co)polymers. U.S. Pat. No. 2,646,417 describes processing aids in the form of styrene interpolymers made from monomeric mixtures containing as the two essential monomers from 50 to 90% by weight of styrene and from 10-50% by weight of an acrylic nitrile, in which other monolefinic monomers like ethylacrylate, vinyl acetate, vinyl benzoate, methyl methacrylate may be present but only in minor amounts of less than 20% by weight. Said known processing aids are used in a quantity of 1 to 10% by weight to lower the processing temperature of non-plasticized vinyl halide resins. The known processing aids should be "high in molecular weight".
The one example dealing with terpolymers describes a terpolymer of 65% by weight of styrene, 25% acrylonitrile and 10% by weight of ethylacrylate.
U.S. Pat. No. 3,975,315, corresponding with FR-A-2,062,817 describes "copolymers of a styrene containing about 10 to 40% of a bound acrylonitrile or 25 to 75% of an alkyl substituted acrylate or both". Said copolymers should have a "dilute solution viscosity greater than about 1.5". By incorporating 5 to 20 weight parts, per 100 weight parts of a vinylchloride polymer, of said copolymer the vinylchloride when expanded has upon extrusion sufficient hot strength. According to this prior art it is preferred to use copolymers with greater than 50%, even more preferred with 60 to 90 weight percent of styrene and 10 to 40 weight percent of acrylonitrile, more preferably 15 to 35%. The styrene copolymer may also contain an alkyl methacrylate or ethacrylate in addition to, or in place of, the nitrile in amounts from 5 to 75%, preferably 40 to 60%. The "dilute solution viscosity" of the copolymer is preferably greater than 2.5. It seems to have been determined in a 1% monochlorobenzene solution at a non-disclosed temperature.
FR-A-1.600.621 mentions in very general terms the possibility to use high molecular weight methyl-methacrylate-styrene-acrylonitrile terpolymers as an agent to make polymers more suitable for expansion.